Investigations on the ecology and production-biology of the "floating meadows" (Paspalo-Echinochloetum) on the Middle Amazon. Part II. The aquatic fauna in the root zone of floating vegetation

Besides their importance for primary production, (JUNK 1970) the floating vegetation in the varzea region of the Amazon must be taken into consideration as a very important factor for the secondary production in the waters of this region as well. The floating stands represent a biotope which, with few exceptions, offers very good living conditions for aquatic animals. Accordingly they are populated with a large number of individuals and species. The maximum values found using a net of mesh size 223 microns were 780 000 individuals per m² which is equal to 11.6 g dry weight /m² (62 g fresh weight). The plants are used by the animals as a substratum and partly for nutrition. Experiments with excelsior as a substratum have shown that phytoplankton and detritus, trapped in the roots, are very important for nutrition for the population. High quantities of inorganic suspended material in combination with current (flowing white water) strongly reduce the number of individuals especially the filtering forms. Under such conditions, Cyclestheria hislopi BAIRD (Fam. Conchostraca) was not found. In dense floating islands, which are several years old, the oxygen content can be the limiting factor for the population of aquatic animals. In general we can say that the development of the aquatic fauna is influenced more by local than regional factors. Because of the complex composition of the fauna it can be shown that the short term variations of the ecological factors are shown better by the number of individuals than by the biomass. Animals with a fast succession of generations react to changes in the ecological conditions, and have a greater influence on the number rather than the biomass of individuals, due to their small size. From the point of view of the colonization by aquatic invertebrate fauna the floating vegetation could be divided into three different types, which are briefly characterized in Table 12. The factor which lead to this division and their effects on the animals have been investigated and discussed extensively. For further clarification of the relationship between fauna and substratum, experiments have been conducted with an artificial substratum, excelsior